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Chapter 2
Network Models
2.1
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2-1 LAYERED TASKS
We use the concept of layers in our daily life. As an
example, let us consider two friends who communicate
through postal mail. The process of sending a letter to a
friend would be complex if there were no services
available from the post office.
Topics discussed in this section:
Sender, Receiver, and Carrier
Hierarchy
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Figure 2.1
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Tasks involved in sending a letter
2-2 THE OSI MODEL
Established in 1947, the International Standards
Organization (ISO) is a multinational body dedicated to
worldwide agreement on international standards. An ISO
standard that covers all aspects of network
communications is the Open Systems Interconnection
(OSI) model. It was first introduced in the late 1970s.
Topics discussed in this section:
Layered Architecture
Peer-to-Peer Processes
Encapsulation
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IBM 3090 Mainframe
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IBM 3090, Disk Controllers + Disks
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IBM Terminal Controller
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IBM Terminal
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IBM Modem – 9600 BPS
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Note
The aim: Unite these networks!
ISO is the organization.
OSI is the model.
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Note
Everyone thought OSI would be the norm,
but TCP/IP became the norm.
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Figure 2.2 Seven layers of the OSI model
Intermediate
nodes only use 3
first layers
Please Do Not Take Salami Pizza Away!
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Figure 2.3 The interaction between layers in the OSI model
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Figure 2.4 An exchange using the OSI model
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2-3 LAYERS IN THE OSI MODEL
In this section we briefly describe the functions of each
layer in the OSI model.
Topics discussed in this section:
Physical Layer
Data Link Layer
Network Layer
Transport Layer
Session Layer
Presentation Layer
Application Layer
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Figure 2.5 Physical layer
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Note
The physical layer is responsible for movements of
individual bits from one hop (node) to the next.
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Figure 2.5 Physical layer
Coordinates functions required to carry a bit stream
Concerned with:
 Physical characteristics of interfaces (medium),
 representations of bits,
 Data rate,
 Synchronization of bits,
 line configuration,
 Physical topology,
 transmission mode.
Normally implemented in HARDWARE – unlike the other
protocols...
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Figure 2.6 Data link layer
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Note
The data link layer is responsible for moving
frames from one hop (node) to the next.
….Also finds the physical destination on the net, and decides
who's turn it is to talk now (called “bus arbitration”)!
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Data Link Layer Responsible for:
Framing (dividing bits into units)
Physical addressing
Flow control
Error control
Access control (which device has control of the link)
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Figure 2.7 Hop-to-hop delivery
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Figure 2.8 Network layer
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Note
The network layer is responsible for the
delivery of individual packets from
the source host to the destination host (maybe over different
networks).
..tries to find the shortest path between them..
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Responsible for source to destination of a packet
Also:
Logical addressing
Routing
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Figure 2.9 Source-to-destination delivery
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Figure 2.10 Transport layer
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Note
The transport layer is responsible for the delivery
of a message from one process to another.
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Transport layer does:





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Service point addressing (port address)
Segmentation and reassembly
Connection control
Flow control
Error control
Figure 2.11 Reliable process-to-process delivery of a message
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Figure 2.12 Session layer
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Note
The session layer: Responsible for Login, Permissions,
Rights.
(also responsible for session checkpointing and recovery,
example application is web conferencing, in which the
streams of audio and video must be synchronous to avoid socalled lip synch problems.
Another application is in live TV programs, where streams
of audio and video need to be seamlessly merged and
transitioned from one to the other to avoid silent airtime or
excessive overlap.)
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Figure 2.13 Presentation layer
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Note
The presentation layer is responsible for translation,
compression, and encryption.
example of a presentation service would be the conversion of
an EBCDIC-coded text computer file to an ASCII-coded file.
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Figure 2.14 Application layer
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Note
The application layer is responsible for
providing services to the user.
Provides network-aware applications that provide
Network virtual terminal, file transfer, access and
management, email, directory services.
Examples: Email, Web Browser, File Sharing, Printer
Sharing, etc!
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Figure 2.15 Summary of layers
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2-4 TCP/IP PROTOCOL SUITE
The layers in the TCP/IP protocol suite do not exactly
match those in the OSI model. The original TCP/IP
protocol suite was defined as having four layers: host-tonetwork, internet, transport, and application. However,
when TCP/IP is compared to OSI, we can say that the
TCP/IP protocol suite is made of five layers: physical,
data link, network, transport, and application.
Topics discussed in this section:
Physical and Data Link Layers
Network Layer
Transport Layer
Application Layer
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Figure 2.16 TCP/IP and OSI model
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OSI Data Link+Physical = TCP “Network Access Layer”
2-5 ADDRESSING
Four levels of addresses are used in an internet employing
the TCP/IP protocols: physical, logical, port, and specific.
Topics discussed in this section:
Physical Addresses : (Example : MAC Addresses)
Logical Addresses : (IP Addresses)
Port Addresses
Specific Addresses: (Email addresses, Domain Names, etc)
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Figure 2.17 Addresses in TCP/IP
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Figure 2.18 Relationship of layers and addresses in TCP/IP
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Example 2.1
In Figure 2.19 a node with physical address 10 sends a
frame to a node with physical address 87. The two nodes
are connected by a link (bus topology LAN). As the
figure shows, the computer with physical address 10 is
the sender, and the computer with physical address 87 is
the receiver.
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Figure 2.19 Physical addresses
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Example 2.2
As we will see in Chapter 13, most local-area networks
use a 48-bit (6-byte) physical address written as 12
hexadecimal digits; every byte (2 hexadecimal digits) is
separated by a colon, as shown below:
07:01:02:01:2C:4B
A 6-byte (12 hexadecimal digits) physical address.
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Example 2.3
Figure 2.20 shows a part of an internet with two routers
connecting three LANs. Each device (computer or
router) has a pair of addresses (logical and physical) for
each connection. In this case, each computer is
connected to only one link and therefore has only one
pair of addresses. Each router, however, is connected to
three networks (only two are shown in the figure). So
each router has three pairs of addresses, one for each
connection.
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Figure 2.20 IP addresses
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Example 2.4
Figure 2.21 shows two computers communicating via the
Internet. The sending computer is running three
processes at this time with port addresses a, b, and c. The
receiving computer is running two processes at this time
with port addresses j and k. Process a in the sending
computer needs to communicate with process j in the
receiving computer. Note that although physical
addresses change from hop to hop, logical and port
addresses remain the same from the source to
destination.
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Figure 2.21 Port addresses
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Note
The physical addresses will change from hop to hop,
but the logical addresses usually remain the same.
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Example 2.5
As we will see in Chapter 23, a port address is a 16-bit
address represented by one decimal number as shown.
753
A 16-bit port address represented
as one single number.
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